1. Field of the Invention
The present invention relates to a route guide apparatus such as a navigation system and the like, mounted on a vehicle such as an automobile.
2. Description of the Related Art
Many conventional navigation systems have a route search function for searching for a route leading to a destination based on the road network information to minimize the distance to be covered and the time required, and displaying results of the search on a display unit together with a road map. Guide points, such as intersections at which the traveling direction is to be changed, are indicated along the guide route thus searched, so that a vehicle passing a given guide point is informed in more detail than at other points.
In order to guide the running vehicle smoothly, it is necessary to supply a guidance at each guide point before the vehicle reaches the particular guide point. This is because the vehicle may be required to turn at an intersection, for example, and must change the lane and decelerate before reaching the intersection. On the other hand, in urban areas, roads are so densely built that a guide point such as an intersection, even if the vehicle driver is informed of it before the guide point, often cannot be identified easily. A technique for supplying a guidance at a point back from a predetermined intersection along a guide route is disclosed in Japanese Unexamined Patent Publications JP-A-7-49243 (1995) and JP-A-7-103776 (1995), in which "the distance to be covered before reaching an intersection at which to turn" and "the number of intersections to be passed before reaching an intersection at which to turn", respectively, are announced by voice.
In running through an area where roads are densly-built, the driver may often find it difficult to recognize an intersection of a guide point, if the intersection has been identified exclusively by a distance to be covered before reaching the intersection, as in the prior art of Japanese Unexamined Patent Publication JP-A 7-49243 (1995). This is because densely-built roads and a great number of intersections makes it necessary to change traveling speeds frequently, making ir difficult to judge the distance grasping the sense of distance. On the other hand, in Japanese Unexamined Patent Publication JP-A 7-103776 (1995), when roads are congested with traffic or the like, it is difficult for the driver informed of the number of intersections to recognize each intersection visually. In many such cases, the driver has no choice but to recognize each intersection by viewing a traffic signal such as a traffic light provided at the intersection. However, not every intersection is not provided with a traffic light. In the case where the number of traffic lights is not consistent with the number of intersections, the driver may not be correctly informed of the location of the guide points.
There exist other Japanese Unexamined Patent Publications JP-A 6-68386 (1994) and JP-A 7-35559 (1995), in which traffic signals are displayed on a road map for guiding the driver along a route. In Japanese Unexamined Publication JP-A 6-68386, it is suggested that a route guidance may be announced by voice, but it is not suggested to supply a guidance before a particular guide point. In Japanese Unexamined Publication JP-A 7-35559, traffic signals are displayed on a route map with other items so that it could be very difficult for a driver to count the number of traffic signals existing on the route driving.